In recent diesel internal combustion engines mounted on vehicles, a particulate filter is installed on a way of an exhaust pipe to collect particulate matters so as to reduce the particulate matters contained in an exhaust gas.
The particulate filter is generally made of porous ceramics having a multiplicity of exhaust gas passages. When the exhaust gas passes through porous filter walls that partition the exhaust gas passages, the particulate matters are adsorbed and collected. If the particulate matters collected in the particulate filter keep accumulating in the particulate filter, a pressure loss in the particulate filter may increase and engine efficiency may decrease. Therefore, the particulate filter is regenerated by burning the particulate matters at adequate timings.
Specifically, the particulate filter supports an oxidation catalyst therein. In a regeneration of the particulate filter, a post fuel injection is performed after a main fuel injection to supply unburned HC (hydrocarbon) to the particulate filter. Then, an internal temperature of the particulate filter is increased by a catalysis of the unburned HC so as to burn and remove the particulate matters in the particulate filter (refer to US-2003-0230078-A1 and its counterpart JP-2004-019496-A, for example).
In an above-mentioned exhaust emission control system according to US-2003-0230078-A1 and its counterpart JP-2004-019496-A, HC is burned by the catalyst supported in the particulate filter in a normal regeneration of the particulate filter. Thus, as shown in FIG. 2, an exhaust gas temperature increases to a burning temperature of the particulate matters, to burn and remove the particulate matters in the particulate filter.
However, as shown in FIG. 2, in the normal regeneration of the particulate filter, the catalysis of the unburned HC, which is supplied by the post injection, does not occur enough in a front end portion of the particulate filter. Thus, the exhaust gas temperature does not increase enough to burn the particulate matters, so that some of the particulate matters accumulated in the front end portion remain unburned.
If this problem continues, a regeneration deficiency occurs in the front end portion of the particulate filter. Specifically, as shown in FIG. 3, the particulate matters accumulate in the front end portion to clog the exhaust gas passages in the particulate filter. The clogging can enlarge a pressure loss in the particulate filter and decrease an output power of the engine.
If a regeneration process is performed to the particulate filter in which the particulate matters are densely accumulated in the front end portion, the densely accumulated particulate matters can burn rapidly, to cause an excessively large temperature increase to damage the particulate filter.